To detect an open/close state of a fluid flow path, there has been known a device which detects an upper end position and a lower end position of a valve stem using limit switches. Although such an opening degree detection device can detect a fully open state or a fully closed state, the opening degree detection device has a drawback that the opening degree detection device cannot detect an intermediate state such as the opening degree of 5% or the opening degree of 95%. In view of the above, patent literature 1 proposes an opening degree detection device for an automatically operated valve where, using a displacement sensor fixed to a casing of an automatically operated valve and a tapered cylindrical target integrally mounted on a valve stem, the opening degree of the fluid flow path is detected based on a value measured by the displacement sensor which is arranged to face an inclined detection surface (tapered cylindrical surface) of the target which moves along with the upward and downward movement of the valve stem.